Amano, addressing a press briefing on Monday, said all of Japan's nuclear material is under the IAEA's safeguard. There was no sign that nuclear material in Japan "has the risk of being diverted" for military applications.

Qin said China has taken note of the IAEA chief's comments, but the statement had failed to reassure and has raised the doubt and concern of the international community.

"Has Japan kept an excessive amount of sensitive nuclear material that is beyond its actual needs? Does one need so much sensitive nuclear material for peaceful use? Should one keep excessive weapons-grade nuclear material?" asked the spokesperson.

"More importantly, does Japan have higher-enriched and weapons-grade uranium, and how much does it have? What are those used for? How can Japan ensure a balance between the demand and supply of nuclear materials? These are the real concerns and questions of the international community," Qin said.

Avoiding answering questions does not help to solve the problem, he said, urging Japan to face up to the issue and give explanations as soon as possible.

"We also hope the IAEA will take a fair and objective attitude on this matter, clarify the issue of Japan's excessive nuclear stockpile, and request that the relevant country solve the issue," he said.

The United States offered 331 kilograms of weapons-grade plutonium to Japan during the Cold War, Japan's Kyodo News Agency reported in late January. The highly concentrated plutonium, which is kept by Japan's atomic energy agency, could be used to produce 40 to 50 nuclear weapons. Japan holds another 44 tons of plutonium, which could be used for nuclear reactors.

The United States has been pressing Japan to return the nuclear material since the first nuclear security summit in 2010. The two sides are expected to reach an agreement at the third summit in the Netherlands this March.